It is known for electronic devices, such as mobile telephone handsets and the like, to comprise audio functionality. Traditionally, such electronic devices comprise traditional electromagnetic speaker components that use a coil and a cone or diaphragm to convert electrical signals into audio signals. Manufacturers of such electronic devices are continually striving to reduce weight and size of the devices, whilst increasing functionality, in order to meet market demands and provide a competitive advantage. As will be appreciated by a skilled artisan, the use of a cone and coil within traditional electromagnetic speakers results in these speakers being relatively bulky. Thus, traditional electromagnetic speakers tend to be one of the most problematic features when trying to minimise the size and weight of devices incorporating such traditional speaker components.
Piezo-ceramic flat speakers are known, to use a ceramic disk glued to a membrane to convert electrical signals into audio signals. Consequently, such piezo-ceramic flat speakers comprise a substantially reduced thickness, compared to traditional electromagnetic speakers, for example in a region of 0.8 mm, which is approximately one fifth that of the thinnest traditional electromagnetic speakers. Furthermore, such piezo-ceramic flat speakers are light weight. Advancements in piezo-ceramic technology have resulted in piezo-ceramic speakers being capable of meeting the standards of audio reproduction required for electronic devices, such as mobile telephone handsets, whilst also enabling a reduction in size and weight of such devices. Piezo-ceramic speakers also provide the additional benefits of low energy requirements at low frequencies, and higher acoustic output compared to electromagnetic speakers (in the order of 60%).
Class D amplifiers are well known in the art, and are generally deemed suitable for driving electromagnetic speakers. Class D amplifiers use switching technology to achieve high power efficiency. In particular, electromagnetic speakers typically present a relatively high impedance (e.g. 100Ω) at the operating frequency of the Class D amplifier (e.g. 1 MHz). However, the impedance of piezo-ceramic speakers is a purely capacitive reactance. Consequently, Class D amplifiers are unsuitable for driving piezo-ceramic speakers, since their impedance is close to that of a short circuit at the operating frequency of a Class D amplifier.
Instead, linear amplification design is required to drive piezo-ceramic speakers, such as provided by a Class AB amplifier.
FIG. 1 illustrates an example of conventional amplifier circuitry 100 for converting a single ended signal at an output of a Programmable Gain Amplifier (PGA) 110 into a differential signal applied to a speaker 120, such as a piezo-ceramic speaker. The amplifier circuitry 100 comprises a Single to Differential (S2D) amplifier circuit 130 and two single ended Class AB linear power amplifier circuits 140, 150. Unfortunately, for electronic devices such as mobile telephone handsets, the use of three amplifier circuits requires a large silicon area, which is undesirable when silicon area is at a premium, due to a need to reduce the cost of such devices to meet market demands.
In addition, each amplifier circuit contributes independently to the output noise of the amplifier circuitry, resulting in a relatively high noise floor. Furthermore, the use of three amplifiers circuits results in a large quiescent current, requiring complex quiescent current control circuitry. Consequently, the use of three amplifier circuits, in this manner, to provide the linear amplification required for piezo-ceramic speakers is undesirable.
A further problem with conventional single ended Class AB linear power amplifier circuits 140, 150 is that their input stages are typically required to have rail-to-rail capability, e.g. capability for the voltage of the output nodes of the input stage to reach the levels of supply and ground rails in order to efficiently drive the power transistors of the output stages. As a result, a folded-cascode structure, as illustrated in FIG. 1B, is typically used for the input stage, resulting in a high component count as well as requiring additional silicon area. A folded-cascode structure is a differential amplifier stage in which additional branches allow the output node to have a larger voltage swing capability.
WO1991/007814 describes an amplifier that has a folded-cascode input stage, an AB class output stage, a first common mode feedback circuit that stabilizes the input stage and a second common mode feedback circuit that stabilizes the output stage. Ten capacitors are needed for frequency compensation, which results in a very area-consuming solution. This amplifier is not adapted to low supply voltages (it is supplied with 5V) because the class AB function is achieved due to a transconductance stage that requires a relatively large voltage headroom. Quiescent current control is achieved by component matching only and there is no minimal current regulation, which is detrimental to the Total Harmonic Distortion.
Thus, a need exists for an improved semiconductor device with an amplifier circuit where at least some of the aforementioned disadvantages with prior art arrangements are substantially alleviated.